The present invention relates to systems which automatically regulate the transport of longitudinally transported strip material, especially large-width photographic roll paper transported through a copying machine, in such a manner that sensors at the lateral edges of the transported strip detect transverse shifting of the strip and initiate corrective recentering action performed by position-correcting means.
In a great variety of practical applications, it is necessary to assure that the longitudinal transport of strip material is performed with great exactness. This is an important concern, for example, in photographic copying or printing machines, which must be capable of handling paper strips of various different breadths up to a breadth of about 30 cm or more. If the transported strip is quite narrow, then rigid, stationary transverse or lateral guide structures may often be adequate, depending upon the type of strip stock employed. However, generally, if the breadth of the strip stock exceeds about 10 cm, stationary lateral guide structures are no longer adequate; instead of providing lateral guidance and positioning force, applied to the strip transverse to its transport direction, the lateral guide structure merely deform the edge portions of the strip, due to the strip's tendency to curl up and/or due to its relatively low stiffness.
It is known, for example from U.S. Pat. No. 3,001,680, to provide sensing rollers at the lateral edges of the transported strip. The sensing rollers sense transverse shifting of the transported strip and, through the intermediary of a lever coupling, activate the drive motor for a transverse-shifting structure serving to shift the transported strip transversely back to centered position. Such known systems, however, are quite expensive and complicated in their operation. A particular disadvantage of them, is that the transverse-shifting structure used to return the strip to centered position presses on the edges of the strip; if the strip has a marked tendency to roll or curl up, or is otherwise very flexible, little or no corrective transverse shifting may actually result. In particular, photographic roll paper of the type nowadays typically empolyed cannot be adequately centered using such systems; the inherent stiffness of the paper is so low that its edges cannot in general follow along the recentering structure, and instead may often merely become deformed. Furthermore, such deformation, i.e., the pressure applied to the strip in the course of such deformation, can result in improper exposure of the strip at its edges.